News Release Number: STScI-2005-02

Light Continues to Echo Three Years After Stellar Outburst

February 3, 2005: The Hubble Space Telescope's latest image of the star V838 Monocerotis
(V838 Mon) reveals dramatic changes in the illumination of surrounding
dusty cloud structures. The effect, called a light echo, has been
unveiling never-before-seen dust patterns ever since the star suddenly
brightened for several weeks in early 2002.

Q & A: Understanding the Discovery

1.
What is a light echo?

It is light from a stellar explosion echoing off dust surrounding the
star. V838 Monocerotis produced enough energy in a brief flash to
illuminate surrounding dust, like a spelunker taking a flash picture of
the walls of an undiscovered cavern. The star presumably ejected the
illuminated dust shells in previous outbursts. Light from the latest
outburst travels to the dust and then is reflected to Earth. Because of
this indirect path, the light arrives at Earth months after light from
the star that traveled directly toward Earth.

2.
Why did the star produce this outburst?

Astronomers do not fully understand the star's outburst. It was
somewhat similar to that of a nova, a more common stellar outburst. A
typical nova is a normal star that dumps hydrogen onto a compact white-
dwarf companion star. The hydrogen piles up until it spontaneously
explodes by nuclear fusion  like a titanic hydrogen bomb. This
exposes a searing stellar core, which has a temperature of hundreds of
thousands of degrees Fahrenheit.

By contrast, V838 Monocerotis did not expel its outer layers. Instead,
it grew enormously in size. Its surface temperature dropped to
temperatures that were not much hotter than a light bulb. This behavior
of ballooning to an immense size, but not losing its outer layers, is
very unusual and completely unlike an ordinary nova explosion.

The outburst may represent a transitory stage in a star's evolution
that is rarely seen. The star has some similarities to highly unstable
aging stars called eruptive variables, which suddenly and unpredictably
increase in brightness.

3.
Are light echos similar to sound echos?

The echoing of light through space is similar to the echoing of sound
through air. As light from the stellar explosion continues to propagate
outwards, different parts of the surrounding dust are illuminated, just
as a sound echo bounces off of objects near the source, and later, objects
further from the source. Eventually, when light from the back side of the
nebula begins to arrive, the light echo will give the illusion of contracting,
and finally it will disappear.

4.
Where is V838 Mon located?

V838 Mon is located about 20,000 light-years away from Earth in the direction
of the constellation Monoceros, placing the star at the outer edge of our
Milky Way galaxy.